This application relates to optical sensing including optical sensing of chemical and biological substances.
Plasmons are eigenmodes of collective density oscillations of quasi-free electrons or an electronic gas in metals and other materials under optical excitation. Plasmons are generated by coupling photons and electrons at or near a surface of an electrically conductive material and thus are sometimes referred to as surface plasmon polaritons (SPPs). The coupling of the photon and electron gas can lead to effective binding energy or a momentum mismatch which precludes coupling of a free space photon to the SPP in normal circumstances. Typically, an incident photon needs some additional momentum to excite a SPP under a phase-matched surface plasmon resonance (SPR) condition.
Surface plasmon polaritons have been extensively studied and some recent work has explored their potential for building various integrated optical devices. The intrinsic mode confinement in SPPs, due to their surface nature, may have potential advantages for building sub-diffraction limited waveguides and in facilitating full three-dimensional optical confinement. Further interest has been sparked by the observation that SPP waves can enhance optical transmittance through optically thick metallic films with sub-wavelength features. The radiated diffraction pattern by excited SPPs can be controlled to operate an SPP device as nano-antennae and transmitters.
Surface plasmon resonance sensors can be constructed for biological and chemical sensing. Many SPR sensors use a metal-dielectric interface and a prism to excite SPP waves via the Kretschmann configuration based on optical evanescent coupling through the prism. In such a SPR sensor, a metallic film is the interrogation medium and is placed or deposited on the prism. The effective numerical aperture of this prism-based system is limited and this further limits the spatial resolution and resolvable spots. In order to meet the SP resonance for a planar metallic film, typical illumination conditions are set at a relatively large angle and this configuration can impose server constraints on the depth of focus in imaging of the system. The limited depth of focus in imaging can be unsuitable for large arrays of assays. In addition, the lateral resolution of the prism-based SP system can be limited by the finite SPP propagation length and are unsuitable for massive parallelization of such SPR sensors.
In 1998, T. E. Ebbessen et al. designed sub-wavelength nanohole arrays in metallic films to produce “extraordinary optical transmission” through such sub-wavelength nanoholes based on excitation of SPPs. See, e.g., Ebbesen et al., Extraordinary Optical Transmission through Sub-Wavelength Hole Arrays, Nature, vol. 39, 667, 669 (1998) and Ghaemi et al., Surface Plasmon Enhance Optical Transmission through Subwavelength Holes, Physical Review, Vol. 58, No. 11, 6779, 6782 (1998). Such nanohole arrays exhibit interesting SPP properties and can be potentially used in various applications.